A semiconductor chip or die (such as an image sensor chip) is typically fabricated on a single semiconductor wafer along with hundreds, and in some cases thousands, of copies of the same die. The cutting needed to separate individual dies from a semiconductor wafer—a process known as “dicing” or “wafer dicing”—can be done with a die saw (such as a diamond saw). Cuts are made along non-functional areas of semiconductor material, known as scribe lines, that separate the dies on the wafer from each other. Using a diamond saw introduces mechanical stress to the semiconductor wafer and can result in cracking at the die edges, compromising the integrity and reliability of the device on the die. An alternative to the diamond saw is laser scribing, which involves scanning a laser beam over the scribe lines of a semiconductor wafer, but this solution has low throughput and the required equipment is expensive.
The increasing demand for image sensors with faster processing speeds and better image quality, while at the same time decreasing the physical size of the image sensor chip, has led to smaller pixel cells sizes and smaller photosensitive regions or photodiodes. Advances in CMOS technology, such as the use of materials with low dielectric constant k, known as low-k dielectrics, that reduce cross coupling and parasitic capacitance between metal layers which make up the metal interconnect, are used to keep up with the decreasing image sensor chip size. However, low-k dielectrics are brittle due to their porous nature, which makes an image sensor with low-k dielectric prone to peeling and cracking when diamond saws are used to separate dies on a semiconductor wafer.
In addition to using low-k dielectric, another way to increase the size of a photodiode in a pixel cell, is to use a backside illuminated (“BSI”) image sensor. BSI image sensors include a pixel array fabricated on a frontside of the semiconductor wafer, but receive light through a back surface of the image sensor. During fabrication of a BSI image sensor, the image sensor chip or device is first fabricated on a semiconductor wafer. When all the necessary elements have been formed in or on the device wafer, the device wafer is bonded to a carrier wafer for further processing. Due to the tradeoff of parameters such as bonding strength and wafer distortion, bonding strength can be maximized, but this can cause weakness at the bonding interface. This combination of low-k dielectric in the device wafer and weak bonding interface can increase the occurrence of peeling and cracking when a diamond saw is used to dice BSI dies from the combined wafer (device wafer plus carrier wafer).